1. Field of the Invention
The present invention relates to a pulse generating circuit in which the amplitude of the output pulse is variable. The present invention is particularly applicable to a supersonic pulse wave generator used for a medical diagnosis device, flaw detector, sonar and the like and is also applicable to other devices using amplitude variable pulses.
2. Description of the Related Art
Supersonic diagnosis equipment is provided with a scanning head which radiates a supersonic wave and detects the wave reflected from various parts of the object. The head is provided with a vibrator which transmits the supersonic wave when supplied with an electric driving pulse. The head is typically provided with a plurality of vibrators each driven independently to form a supersonic wave beam which allows the beam to be scanned or focused at a desired point. A pulse generator, for driving each supersonic vibrator, is required which has the capability of varying the amplitude of the output driving pulse provided to the corresponding vibrator. Supersonic wave generators for devices such as flaw detectors, sonar, and the like need similar driving pulse amplitude control capability.
FIG. 1 is a block diagram of a typical prior art supersonic pulse generator using a piezoelectric vibrator. A central processing unit (CPU) 1, which controls the overall operation of the device, sends out a timing pulse 2 to a pulse generator 3, which outputs a driving pulse 4 to drive a piezoelectric vibrator 5. The CPU 1 also controls a variable voltage source 6 which supplies a voltage to the pulse generator 3. The amplitude of the driving pulse 4 is varied by the voltage supplied to the pulse generator 3 from the variable voltage source 6. The piezoelectric vibrator 5 outputs a supersonic wave, the shape of which is determined by the size and cutting or shape of the piezoelectric element. The amplitude of the supersonic wave is proportional to the amplitude of the driving pulse 4, and hence it is proportional to the output voltage of the variable voltage source 6.
FIG. 2(a) is a prior art pulse generating circuit used for the supersonic wave generator 6 of FIG. 1. An amplifier 7 supplies an amplified pulse to a switching element 8 which turns on and off responsive to the input signal voltage. Usually, a MOSFET (metal oxide semiconductor type field effect transistor) is used as the switching element 8 of FIG. 2(a), however, the switching element 8 may be any other type of device such as bipolar transistor, even though it is more practical to use a MOSFET with respect to cost, size and speed of operation. The piezoelectric vibrator 5 may be any type, such as a magnetostrictive type vibrator. The MOSFET 8 turns on or off state depending on the voltage applied to the gate G. The drain D is connected to a voltage source V.sub.D1 via a resistor 9, while the source S of the MOSFET 8 is grounded. A bipolar transistor 10 is connected between the voltage source V.sub.D1 and the piezoelectric vibrator 5. The collector of the bipolar transistor 10 is connected to the voltage source V.sub.D1, and the base is connected to the drain D of the MOSFET 8. A diode 11 is connected between the emitter and base of the bipolar transistor 10 to supply a current in a direction from the emitter of the transistor 10 to the drain D of the MOSFET 8. A capacitor 12 is connected between the emitter of the bipolar transistor 10 and one end of the piezoelectric vibrator 5, while the other end of the piezoelectric vibrator 5 is grounded.
During operation, first, the capacitor 12 is charged up to the voltage of the voltage source V.sub.D1 through the bipolar transistor 10. When a timing pulse as in FIG. 2(b) is fed to the amplifier 7, it outputs an amplified pulse where the polarity of the output pulse of the amplifier 7 is inverted as shown in FIG. 2(c), and the amplitude of the pulse is substantially equal to the supply voltage V.sub.cc of the amplifier 7. This output pulse is fed to the gate G of the MOSFET 8, and it turns the MOSFET 8 on. As a result, the charge in the capacitor 12 is discharged to ground through diode 11 and MOSFET 8. This discharge causes the voltage at one end of the piezoelectric vibrator 5 to drop as shown in FIG. 2(d). At the trailing edge of the pulse, the gate voltage of the MOSFET 8 is pulled down as shown in FIG. 2(c), and the MOSFET 8 turns off. As a consequence, the capacitor 12 is again charged from the voltage source V.sub.D1 through the bipolar transistor 10. This makes the voltage applied to the piezoelectric vibrator 5 rise as shown in FIG. 2(d). In this way, a voltage pulse is applied to the piezoelectric vibrator 5. The amplitude of the pulse appearing on one end of the piezoelectric vibrator 5 is substantially equal to the voltage of the voltage source V.sub.D1, and if the voltage of V.sub.D1 is varied, the amplitude of the output pulse varies.
A problem occurs when the voltage V.sub.D1 is decreased to generate a very low supersonic wave pulse. When the voltage of the voltage source V.sub.D1 is low, the output pulse of the MOSFET 8 is disadvantageously rounded as shown in FIG. 2(e). The rounding occurs especially at the trailing edge of the pulse. Accordingly, if it is necessary to supply the piezoelectric vibrator 5 with low amplitude but very fast repeating pulses, the trailing edge of the preceding pulse will be superposed on the rising edge of the succeeding pulse. This decreases the resolution of the supersonic diagnosis equipment when, for example, a scan of an organ which is located very close to the surface of a human body occurs.
The above description is a general description of the operation of recent supersonic wave diagnosis equipment. A further description of which can be obtained from for example, Japanese Laid Open Patents:
59-12620 "Pulse Amplifier Circuit" by T. Kawada et al., Jan. 23, 1984; PA0 59-15327 "Pulse Amplifier Circuit" by T Kawada et al., Jan. 26, 1984; or PA0 62-281929 "Supersonic Diagnosis Equipment" by N. Norichika et al., Dec. 7, 1987.